Rare earth sintered magnet and method for production thereof

Abstract

A rare-earth sintered magnet with excellent corrosion resistance and sinterability and a method for producing such a magnet are provided. The rare-earth sintered magnet includes an R 2 T 14 Q type tetragonal compound (where R is at least one rare-earth element, T is at least one transition metal element always including Fe, and Q is boron and/or carbon) as a main phase and a grain boundary phase surrounding the main phase. The R 2 T 14 Q type tetragonal compound as the main phase includes Cr, which substitutes for a portion of Fe, and carbon, which substitutes for a portion of boron, as respective essential elements. The concentration of carbon in the main phase is higher than that of carbon in the grain boundary phase.

1 . A rare-earth sintered magnet comprising an R 2 T 14 Q type tetragonal compound (where R is at least one rare-earth element, T is at least one transition metal element always including Fe, and Q is boron and/or carbon) as a main phase and a grain boundary phase surrounding the main phase, 
 wherein the R 2 T 14 Q type tetragonal compound as the main phase includes Cr, which substitutes for a portion of Fe, and carbon, which substitutes for a portion of boron, as respective essential elements, and    wherein the concentration of carbon in the main phase is higher than that of carbon in the grain boundary phase.    
   
   
       2 . The rare-earth sintered magnet of  claim 1 , wherein 50 at % to 90 at % of the overall grain boundary phase is Co.  
   
   
       3 . The rare-earth sintered magnet of  claim 2 , wherein the grain boundary phase includes an R 3 Co compound.  
   
   
       4 . The rare-earth sintered magnet of one of  claim 1 , wherein the R 2 T 14 Q type tetragonal compound as the main phase further includes Co as another essential element that substitutes for a portion of Fe.  
   
   
       5 . The rare-earth sintered magnet of  claim 1 , comprising: 
 12 at % to 18 at % of R,    60 at % to 88 at % of T,    0.1 at % to 2.4 at % of Cr,    0.5 at % to 13 at % of B, and    0.4 at % to 4.5 at % of C.    
   
   
       6 . A rare-earth sintered magnet comprising an R 2 T 14 Q type tetragonal compound (where R is at least one rare-earth element, T is at least one transition metal element always including Fe, and Q is boron and/or carbon) as a main phase and a grain boundary phase surrounding the main phase, 
 wherein the R 2 T 14 Q type tetragonal compound has a natural electrode potential of −0.75 V or more.    
   
   
       7 . The rare-earth sintered magnet of  claim 6 , wherein a difference in natural electrode potential between the R 2 T 14 Q type tetragonal compound and the grain boundary phase is at most 0.6 V.  
   
   
       8 . A method for producing a rare-earth sintered magnet, the magnet including an R 2 T 14 Q type tetragonal compound (where R is at least one rare-earth element, T is at least one transition metal element always including Fe, and Q is boron and/or carbon) as a main phase and a grain boundary phase surrounding the main phase, the method comprising the steps of: 
 preparing a powder of a main phase alloy, at least 50 vol % of which is the R 2 T 14 Q type tetragonal compound and which includes Cr, boron and carbon as respective essential elements, and a powder of a liquid phase alloy including R and Co; and    sintering the powders, thereby making a rare-earth sintered magnet in which the concentration of carbon in the main phase is higher than that of carbon in the grain boundary phase.    
   
   
       9 . The method of  claim 8 , wherein the main phase alloy includes: 
 11 at % to 16 at % of R,    60 at % to 87 at % of T,    0.2 at % to 2.5 at % of Cr,    1 at % to 14 at % of B, and    0.5 at % to 5.0 at % of C.    
   
   
       10 . The method of  claim 9 , wherein a first alloy including 0.8 mass % to 1.0 mass % of Q and a second alloy including 1.2 mass % to 1.4 mass % of Q are used as the main phase alloy.  
   
   
       11 . The method of  claim 9 , wherein the liquid phase alloy includes: 
 60 at % to 80 at % of R and    20 at % to 40 at % of Co.    
   
   
       12 . The method of  claim 8 , wherein the ratio of the liquid phase alloy to the sum of the main phase and liquid phase alloys is defined within the range of 2 vol % to 20 vol %.  
   
   
       13 . The method of  claim 8 , further comprising the steps of: 
 preparing a melt of a material alloy for the main phase alloy; and    cooling and solidifying the melt of the material alloy at a rate of 100° C./s to 10,000° C./s.